The present invention relates to a television receiver.
In Europe, as a method of television broadcasting, there is known a system called NICAM (Near Instantaneous Compound Audio Multiplex). According to the NICAM system, as shown in FIG. 1, a carrier CV of the video signal, a first audio signal carrier C1, and a second audio signal carrier C2 which have different frequencies compared to a frequency f.sub.cv of the carrier CV of the video signal are provided.
The above first audio signal carrier C1 is a carrier of a frequency modulated analog audio signal. The second audio signal carrier C2 is a carrier of a digitally modulated DQPSK--differential 4-phase PSK PCM audio signal.
The above analog audio signal is a first audio signal SV1 and the signal obtained by modulating the first audio signal carrier C1 by the first audio signal SV1 is a first modulated audio signal SV100. On the other hand, the above PCM audio signal is a second audio signal SV2. The signal obtained by modulating the second audio signal carrier C2 by the second audio signal SV2 is a second modulated audio signal SV200.
An intermediate frequency IF is set to 38.9 MHz. Frequencies f.sub.c1 and f.sub.c2 of the first and second audio signal carriers C1 and C2 are specified as follows. That is, the intermediate frequency IF of 38.9 MHz is set as a reference, and in a direction such that the frequency becomes low. According to the B system of the VHF band and the G system of the UHF band, the frequency f.sub.cl of the first audio signal carrier C1 is set to a position of 5.5 MHz and the frequency f.sub.c2 of the second audio signal carrier C2 is set to a position of 5.85 MHz, respectively. According to the I system, the frequency f.sub.c1 of the first audio signal carrier C1 is set to a position of 6.0 MHz and the frequency f.sub.c2 of the second audio signal carrier C2 is set to a position of 6.552 MHz, respectively.
In the television receiver of the NICAM system, a SAWF (Surface Acoustic Wave Filter: hereinafter referred to as a SAW filter in the specification) is used. Conventional frequency characteristics of the SAW filter are, for instance, as shown by a solid line L2 in FIG. 1. According to the characteristics shown by the solid line L2, the level of the frequency f.sub.c1 of the first audio signal carrier C1 and the level of the frequency f.sub.c2 of the second audio signal carrier C2 are set to be almost equal.
Level ratios of the first and second audio signal carriers C1 and C2 to the carrier CV of the video signal as mentioned above are decided as follows by a standard. That is, assuming that the levels of the carriers CV, C1 and C2 are set to LCV, LC1 and LC2, respectively, it is determined that LCV/LC1=13 dB and LCV/LC2=20 dB. Therefore, the above level ratios are used as input conditions of the SAW filter to detect them.
However, under such input conditions, in the case where the frequency characteristics of the RF signal change by a standing wave ratio (SWR) of the transmission system, and upon demodulation of the second modulated audio signal SV200, the level of the second audio signal carrier C2 decreases, the level ratio (LC1/LC2) increases, and the level of the first audio signal carrier C1 relatively increases. Therefore, an interference of the first audio signal carrier C1 to the second audio signal carrier C2 increases, so that an error rate of the second audio signal SV2 deteriorates. On the other hand, to avoid the interference of the first audio signal carrier C1 to the second audio signal carrier C2, a sensitivity of a detecting circuit to detect the second audio signal SV2 cannot help being lowered.